1. Field of the Invention
The present invention relates generally to silicone emulsions and more specifically to a silicone emulsion composition that is useful as a mold release coating in aluminum die casting.
2. Description of the Prior Art
Silicone emulsion compositions prepared by dispersing organopolysiloxanes uniformly in water with the aid of an emulsifier are widely used as fiber processing agents, mold releasing agents, paint additives, and cosmetic feed materials. Examples of this type of silicone emulsion composition include a silicone emulsion composition made from an aqueous emulsion of dimethylpolysiloxane and polyoxyethylene alkyl phenyl ether sulfuric acid sodium (see Japanese Kokoku Patent No. Sho 53[1978]-13501) and emulsions made from cyclic methylpolysiloxane, emulsifier, and water (see, for instance, Japanese Kokai Patent Application No. Sho 56[1981]-95952.)
Conventional silicone emulsion compositions such as those described above suffer from the drawback that they have a poor mechanical stability. That is to say that under mechanical shear, the emulsion form is easily damaged. For example, when these silicone emulsion compositions are used as a mold release in an aluminum die-casting operation, the silicone emulsion composition is generally diluted several tens of times with water. The diluted solution is fed from a reservoir by a gear pump and sprayed onto the surface of a mold. The water in the emulsion is evaporated therefrom to form a uniform oil film of organopolysiloxane on the surface of the mold. Excess amounts of the diluted solution are recovered for recycling, being fed back by a gear pump. In this repeated liquid feed method, emulsion damage takes place, and it is thereafter impossible to uniformly coat the mold.
The oil film produced by the aforementioned conventional silicone emulsion compositions has an additional drawback in that has a poor paintability. Consequently, when these silicone emulsion compositions are used as a mold releasing agent, the oil film adheres to the surface of the die-cast part and makes painting or marking the part very difficult.
Finally, the oil film produced by the prior art silicone emulsion compositions has a low strength and hence, it has poor lubricity under extreme-pressure. Consequently, when these silicone emulsion compositions are used as a mold releasing agent in aluminum die-cast molding, cracks often form in the oil film produced therefrom. Such cracks cause the aluminum melt to misrun, which produces surface defects in the die cast part.